1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine. More particularly, the invention relates to a control apparatus and a control method for a multiple cylinder internal combustion engine.
2. Description of the Related Art
Although some fuel that is injected from a fuel injection valve into an intake port in an internal combustion engine vaporizes immediately, the rest of the fuel temporarily adheres to the wall surface of the intake port. The fuel that has adhered to the intake port vaporizes from negative pressure inside the intake pipe or the heat from the wall surface of the intake port and, together with vaporized portion of the fuel that has been newly injected from the fuel injection valve, forms an air-fuel mixture. During steady operation, the amount of fuel that is injected from the fuel injection valve and adheres to the intake port is balanced with the amount of fuel adhered to the intake port that vaporizes. Therefore, the air-fuel ratio of the air-fuel mixture that forms inside the cylinder can be made to match a stoichiometric air-fuel ratio by injecting an amount of fuel that corresponds to the stoichiometric air-fuel ratio from the fuel injection valve.
Incidentally, when starting the internal combustion engine, especially during a cold start, the temperature inside the intake pipe and the temperature of the wall surface of the intake port is low, and there is no negative pressure inside the intake pipe. Moreover, not much fuel is adhered to the intake port prior to startup. As a result, a large portion of the fuel that is injected from the fuel injection valve at startup adheres to the intake port. Accordingly, in order to form an air-fuel mixture of an ignitable concentration, a larger amount of fuel must be supplied at least in the initial cycle at startup than that is supplied during steady operation after the engine has warmed up. Also, the fuel is supplied by cylinder, so with a multiple cylinder internal combustion engine that has many cylinders, a large amount of fuel is supplied sequentially to each cylinder. However, when a large amount of fuel is supplied, a corresponding large amount of unburned hydrocarbons (HC) is discharged from the cylinders to the exhaust pipe. Although a catalyst for purifying the exhaust gas is provided in the exhaust pipe, it takes a certain amount of time for the catalyst to activate (or more specifically, for the purifying ability of the catalyst to activate) at startup when the temperature of the catalyst is low. Therefore, it is desirable to suppress the discharge of unburned HC from the cylinders as much as possible at least until the catalyst is activated. Reducing unburned HC produced at startup is a major concern in vehicles that use an internal combustion engine as a source of power.
To date, various technologies have been proposed to address this concern. One such technology is described in Japanese Patent Application Publication No. 8-338282 (JP-A-8-338282) and relates to fuel supply during startup of a multiple cylinder internal combustion engine. As described in JP-A-8-338282, it is not always necessary to supply a large amount of fuel sequentially to each cylinder in order to start a multiple cylinder internal combustion engine. That is, an internal combustion engine can be started even when the supply of fuel to some of the cylinders is stopped. Starting an internal combustion engine while the supply of fuel to some of the cylinders is stopped enables the amount of unburned HC discharged at startup to be significantly reduced. The technology in JP-A-8-338282 is an invention that is based on such knowledge, and determines those cylinders to which fuel should be supplied and those cylinders to which the supply of fuel should be stopped, based on the results of a cylinder determination at startup, and then controls the fuel supply to each of the cylinders according to that determination.
However, from the viewpoint of reducing unburned HC produced at startup of the internal combustion engine, there is room for improvement with the technology described above. In a cylinder to which the supply of fuel is stopped at the beginning of startup, i.e., in a cylinder of which activation is delayed (i.e., a delay activated cylinder), the intake and the exhaust valves are not stopped but instead open and close in conjunction with the rotation of the crankshaft, just like in the cylinders to which fuel is supplied from the beginning of startup (i.e., normally activated cylinders). Therefore, in a delayed start cylinder, mere pumping is performed and only air is discharged from the cylinder into the exhaust pipe. However, this air includes unburned HC that had been adhered to the intake port and the inside wall of the cylinder and the like. As a result, with the technology in JP-A-8-338282, even though activation of some of the cylinders is delayed in order to reduce the amount of unburned HC that is discharged, a large amount of unburned HC may end up being discharged from those delay activated cylinders.